After the success of the original Metal Slug, SNK followed up with the sequel Metal Slug 2 featuring more animation and more vehicles. The third game to be released under the series name is Metal Slug X. It's not a sequel to Metal Slug 2, rather it's more of a reworked version with some improvements.
Metal Slug X takes the term remix literally. If you can imagine taking the various elements in Metal Slug 2, dumping them into a box and shaking them up, then pouring them back out and rearranging them, that's what Metal Slug X is. The level progression is identical, but the enemy population has changed within each. A few new items and power-ups have been added as well.
Players have a choice of the four Metal Slug 2 characters to play with, two male and two female. Starting with a pistol and a stock of grenades, players must guide their characters through the game's levels plowing through the enemy population in each. At the end of levels monstrous bosses await the players, ready to inflict multiple forms of painful deaths to their characters. While moving through the stages players can rescue tied up friendly prisoners of war for power-ups, as well as board vehicles for more firepower.
Rescued prisoners of war drop items that the players can pick up. Some items give point bonuses, but most are weapon upgrades. Players can pick up all the Metal Slug 2 weapons such as the rifle, flame gun, and shotgun as well as a weapon new to the series, an odd thing that shoots a lizard on wheels that hugs the ground and explodes on contact. One prisoner of war, instead of dropping power-ups, follows the player around for the rest of the round shooting homing fireballs at enemies.
The various player usable vehicles, or slugs, in the game give the boarding player more firepower as well as added protection. With the exception of the Camel Slug, all the slug vehicles can take a number of hits before exploding. Optionally players can send their weapons headlong in a suicide attack that causes massive damage but destroys the vehicle.